1. Field of the Invention
This invention relates to a light source device for an endoscope, and more particularly to an improvement in a light source for an endoscope which is used for illuminating the object to be observed and/or photographed by the endoscope.
2. Description of the Prior Art
In the conventional endoscope, it has been known to provide a low luminance illumination when observing the object such as the human stomach and intestines and the like in order not to cause an ulcer by the heat of the light source and to provide a high luminance illumination when taking a photograph of the observed object. When providing the high luminance illumination, a switch is operated to illuminate the object only for a very short time. In this type of conventional endoscope, it is technically very difficult to make the switching operation of the illumination synchronize with the release of the shutter, since the rising characteristic of the illumination is not completely constant. In the endoscope of this type, a camera which determines the shutter speed based on the brightness of the object illuminated by the light source which is measured by integrating the quantity of light reflected by the surface of the film therein, is often used. In this case, it is also difficult to have the start of the integration of the light perfectly synchronize with the start of release of the shutter.
In view of the above problem, it has been proposed to provide a diaphragm in front of the light source of the endoscope to control the illumination without changing the luminance of the light source. In this case, the light source is constantly turned on to provide illumination of high luminance and the diaphragm is operated to stop down the aperture thereof when the object is observed and fully open the aperture thereof when the object is photographed. Further, the diaphragm is closed before the shutter is released and is opened when the shutter is completely opened. The endoscope with the above described diaphragm is advantageous over said conventional endoscope in that the rising characteristic of the illumination does not influence the exposure. However, this kind of endoscope with the diaphragm has a defect in that time is wasted between the initiation of the shutter release and the actual release of the shutter during which the diaphragm is mechanically operated.
The above described endoscope provided with a diaphragm has a further defect as follows. Since the luminance of the light source is constant and the diaphragm is fully opened when the object is photographed, the exposure time is made longer when an object far from the light source of the endoscope is photographed than when an object close thereto is photographed. Therefore, in order to obtain a sharp picture even for the object far from the endoscope, the exposure time is desired to be made as short as possible for the object far from the endoscope. Accordingly, the luminance of the light source is made high. Consequently, the shutter speed of the camera should be made very high for the object close to the endoscope. From a technical viewpoint, it is practically very difficult to obtain such a high shutter speed with a sufficient preciseness and to precisely make the shutter release of such a high speed synchronize with said start of integration of light or operation of the diaphragm.